Cellular network deployments are increasingly heterogeneous, in that they comprise a range of different types of base station. Typical base stations, having a large coverage area, may be referred to as macrocell base stations. Additionally, there may be a dense distribution of smaller cells, called femtocells. These may be created by access points called Home Base Stations (HBTSs). In the context of Third Generation Partnership Project (3GPP) Universal Mobile Telecommunications System (UMTS), these are referred to as Home Node B (HNB) and in the context of 3GPP Long Term Evolution (LTE) as Home eNode B (HeNB).
The femtocells can be deployed on the same carrier frequency or different carrier frequency to the macrocells. However, whereas the deployment of macrocell base stations is normally planned (in other words, the operator provides the configuration to the base station which indicates all the backhaul interface relations with other neighbouring macrocell base stations), the femtocells may be deployed in an uncoordinated manner. In view of the uncoordinated deployment and large number of HBTSs, the Macrocell Base Station (MBTS) is unlikely to be able to maintain a transport connection to all neighbouring HBTSs or HBTSs within the coverage area of the MBTS. Also, HBTSs can restrict access, partially or exclusively, to members of a Closed Subscriber Group (CSG). Where access is partially restricted to a CSG (members of the CSG having preferential usage of the base station's resources), the HBTS can be referred to as a hybrid cell. These issue's can present challenges for handover. Some of these are addressed in 3GPP TS 36.300 10.5.1.2, which is incorporated herein by reference, by enabling mobile stations to provide an MBTS with sufficient information about the HBTSs so that the MBTS can establish a transport connection towards the HBTS and perform the handover.
Load balancing is increasingly a desirable feature for cellular networks. For UMTS and LTE, an implementation is described in 3GPP TS 36.300 22.4.1, which is incorporated herein by reference. Exchanging load information between base stations, termed load reporting, allows load balancing to be effected. Load reporting is implemented over the X2 or S1 interfaces, which are backhaul channels. The use of the X2 and S1 interfaces are natural choices for exchanging the load indicators when the backhaul connections actually exist between the two network entities.
In the case of HBTSs, it is not desirable (or even possible) for the MBTS to maintain an interface with all HBTSs. Thus, in the context of load balancing between MBTS and HBTS, it is not possible to exchange load balancing information between neighbouring nodes as a backhaul connection is not established in most cases.
Moreover, the use of CSG and hybrid cell implementations has further complicated load balancing procedures. This is especially the case in terms of relevant load indicators to provide a true reflection of the spare capacity of the HBTS being used by members and non-members of the cell.